1. Technical Field
The present invention relates to an impedance matching apparatus disposed between a high-frequency power source for supplying a power to a load such as a plasma treating processing apparatus used for plasma etching, plasma CVD, or the like, and the load, and for matching an impedance of the high-frequency power source and that of the load.
2. Related Art
FIG. 5 is a diagram showing an example of a high-frequency power supply system in which an impedance matching device is used.
The high-frequency power supply system is a system for performing processing such as plasma etching, or plasma CVD on a workpiece such as a semiconductor wafer or a liquid crystal substrate, and configured by a high-frequency power source 1, a transmission line 2, an impedance matching apparatus 3, a load connecting portion 4, and a load 5 (plasma treating processing apparatus 5).
The high-frequency power source 1 is an apparatus for outputting a high-frequency power, and for supplying the high-frequency power to the plasma treating processing apparatus 5 which functions as the load 5. The high-frequency power output from the high-frequency power source 1 is supplied to the load 5 through the transmission line 2 formed by a coaxial cable, the impedance matching apparatus 3, and the load connecting portion 4 configured by shielded copper plates. The high-frequency power source 1 of this type generally outputs a high-frequency power of a frequency of the radio-frequency band (for example, a frequency of several hundreds of kHz or higher).
The load 5 (plasma treating processing apparatus 5) is an apparatus comprising a processing portion for processing (etching, CVD, or the like) a workpiece such as a wafer, or a liquid crystal substrate carried into the inside of the processing portion. In order to process the workpiece, a plasma discharge gas is introduced into the processing portion, and the high-frequency power (voltage) supplied from the high-frequency power source 1 is applied to the plasma discharge gas, thereby producing a discharge in the plasma discharge gas to change the condition from a non-plasma condition to a plasma condition. Then, the workpiece is processed by using the plasma.
The impedance matching apparatus 3 matches the impedance of the high-frequency power source 1 and that of the load 5. In the impedance matching apparatus 3, a matching portion 10 (see FIG. 6) including a variable impedance element or the like in which the impedance is variable, such as a variable capacitor, or a variable inductor is provided. The impedance of the variable impedance element of the matching portion 10 (in more detail, a capacitance in the case of a variable capacitor, and an inductance in the case of a variable inductor) is changed, thereby performing the impedance matching.
More specifically, for example, when the impedance viewed from an output terminal of the high-frequency power source 1 toward the high-frequency power source 1 is designed to be 50 Ω, and the high-frequency power source 1 is connected to an input terminal 3a of the impedance matching apparatus 3 via the transmission line 2 having a characteristic impedance of 50 Ω, the impedance matching apparatus 3 converts an impedance Zin (hereinafter, referred to as an input impedance Zin) viewed from the input terminal 3a of the impedance matching apparatus 3 toward the load 5, to 50 Ω.
At this time, for example, a range of 50 Ω±1 Ω is set. When the value of the impedance falls in the range, the impedance matching may be assumed to be attained. The target impedance is not limited to 50 Ω, but another impedance (for example, 51 Ω) can be intentionally set.
In the matching portion 10 of the impedance matching apparatus 3, at least one variable impedance element is disposed. Alternatively, another type of element, such as a capacitor or an inductor in which the impedance is fixed may be disposed.
FIG. 6 shows an exemplary circuit configuration of the matching portion 10.
As shown in FIG. 6, the matching portion 10 is configured by, for example, a first variable capacitor C1, a second variable capacitor C2, and an inductor L1.
The first variable capacitor C1 and the second variable capacitor C2 are capacitors in which the capacitance is variable. The first variable capacitor C1 and the second variable capacitor C2 have movable portions which are not shown in the figure, respectively. When the position of the movable portion is changed, the capacitance can be varied. That is, the first variable capacitor C1 and the second variable capacitor C2 are a type of variable impedance elements. The inductor L1 is an inductor having a fixed inductance disposed between the second variable capacitor C2 and an output terminal 10b. 
As shown in FIG. 1 which will be described later, an input-terminal information detecting portion 20 is disposed between the input terminal 3a of the impedance matching apparatus 3 and an input terminal 10a of the matching portion 10. In the specification, however, electric characteristics at the input terminal 3a of the impedance matching apparatus 3 are assumed to be substantially the same as those at the input terminal 10a of the matching portion 10. When the impedance viewed from the input terminal 3a of the impedance matching apparatus 3 toward the load 5 is the input impedance Zin, therefore, the impedance viewed from the input terminal 10a of the matching portion 10 toward the load 5 is also the input impedance Zin.
The output terminal 10b of the matching portion 10 is substantially identical with an output terminal 3b of the impedance matching apparatus 3. When the impedance viewed from the output terminal 3b of the impedance matching apparatus 3 toward the load 5 is an output impedance Zout, therefore, the impedance viewed from the output terminal 10b of the matching portion 10 toward the load 5 is also the output impedance Zout.
In the case of the impedance matching apparatus 3 comprising the above-described matching portion 10, at the input terminal 10a of the matching portion 10 (substantially identical with the input terminal 3a of the impedance matching apparatus 3), both the voltage and the current are not abnormally raised. At the output terminal 10b of the matching portion 10, however, the voltage may be sometimes raised abnormally, as compared with the input terminal 10a. This may cause a discharge trouble. At the output terminal 10b of the matching portion 10, a large current flows, so that a breakage caused by heat generation may occur.
Therefore, an element disposed on the output side of the matching portion 10 such as the second variable capacitor C2 and the inductor L1 is required to be designed so as to withstand a high voltage and a high current. Actually, however, an element having a limited withstand voltage value, and a limited withstand current value in the range which satisfies the specifications required by the customer is selected for the purposes of miniaturization, light-weight, and reduction of production cost. This is because, if an element is selected by estimating a possible maximum value, the size and weight of the element are increased, and also the production cost is increased, so that such an element is impractical.
For this reason, the customer performs the setting of an output power value output from the high-frequency power source 1, or the like so as not to exceed the required specifications. The reason of this is that a voltage and current at the output terminal 10b can be varied by changing the output power value. If the power setting or the like which exceeds the required specifications is erroneously performed by any chance, however, there is the possibility that an element of the matching portion 10 is broken by a voltage exceeding the withstand voltage value of the element, or by a current exceeding the withstand current value.
As a countermeasure against the above, as disclosed in JP-A-6-11528, a voltage at the output terminal 10b is calculated, and the calculated voltage value is monitored. By using this technique, when the calculated voltage value becomes equal to or larger than a predetermined value, it is possible to determine that it is abnormal. In the case where the abnormality is detected, for example, the output of the high-frequency power source 1 is stopped, thereby preventing the element of the matching portion 10 from being broken.
Patent Reference 1: JP-A-6-11528
In the above-described method in which the voltage at the output terminal 10b of the matching portion 10 is calculated, thereby determining whether it is abnormal or not in order to prevent breakage of the element of the matching portion 10 from occurring, the following problems arise.                (1) The voltage at the output terminal 10b is obtained, but the current cannot be uniquely obtained. The current changes also depending on the output impedance Zout. When this method is used, therefore, it is impossible to determine whether a current which exceeds the withstand current value of the element flows or not.        (2) Only the voltage at the output terminal 10b of the matching portion 10 is calculated, so that the abnormality decision for elements existing between the input terminal 10a and the output terminal 10b of the matching portion 10 cannot be performed. In the case of the matching portion 10 shown in FIG. 6, for example, the abnormality determination for the variable capacitor C1 cannot be performed. In addition, as for the variable capacitor C2, it is impossible to determine whether the voltage across applied to the both ends of the element exceeds the withstand voltage value or not.        (3) As described in JP-A-6-11528, the output impedance Zout at the output terminal 10b can be calculated. By applying this method, it is possible to calculate a voltage and a current individually for respective elements of the matching portion 10. However, the impedance of the variable impedance element of the matching portion 10 is not constant, and the output power value and the like of the high-frequency power source are not constant. Therefore, it is necessary to perform the calculation depending on the conditions at the time. However, the calculation requires many calculation steps, so that the calculation during the matching operation requires a large calculation load.        